A pipe which is a component of a solar energy collector or a sea-water desalting unit, and a pipe which is a component of a heat exchanger such as a radiator of an engine or a condenser of a cooler (hereinafter generally referred to as a "pipe") are susceptible to corrosion by fresh water or sea-water flowing therethrough. As a means to prevent the above-mentioned corrosion of the pipe, a method is known, which comprises subjecting the pipe to a cathodic protection by cladding an aluminum alloy excellent in sacrificial anode property onto the inner surface of the pipe.
When a too high flow velocity of liquid flowing through the pipe causes a turbulence in the liquid, the inner surface of the pipe is rapidly corroded under the additional effect of mechanical damage caused by the liquid and corrosion caused by the liquid. Such a phenomenon is called erosion-corrosion. Therefore, an aluminum alloy to be cladded onto the inner surface of a pipe for the purpose of cathodic protection against an erosion-corrosion is required to be excellent not only in sacrificial anode property, but also in erosion-corrosion resistance.
The "Registration Record of International Alloy Designation and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys" (revised as of Sept. 1, 1976) published by the Aluminum Association, Inc., gives on pages 6 to 7 the following list of standards (hereinafter referred to simply as "AA"):
Zinc (Zn) has an effect of improving sacrificial anode property of an aluminum alloy, whereas magnesium (Mg) and manganese (Mn) have an effect of improving erosion-corrosion resistance of an aluminum alloy. The aluminum alloy of the present invention therefore contains Zn, Mg and Mn as indispensable constituent elements as described later.
AA 6253 Al alloy, which contains Zn and Mg, also contains from 0.15 to 0.35 wt.% chromium (Cr) and from 0.45 to 0.975 wt.% silicon (Si) which make an anodic potential thereof noble. As a result, AA 6253 Al alloy is poor in sacrificial anode property.
In AA 7003 Al alloy, containing Zn and Mg, the Zn content is so high as from 5.0 to 6.5 wt.%, and Mn is contained in an amount of only up to 0.30 wt.% as an impurity. AA 7003 Al alloy is therefore insufficient in erosion-corrosion resistance.
AA 7039 Al alloy, although containing Zn, Mg and Mn, is poor in erosion-corrosion resistance because of the high Zn content of from 3.5 to 4.5 wt.% as in the above-mentioned AA 7003. Furthermore, AA 7039 Al alloy is also insufficient in sacrificial anode property since said alloy contains from 0.15 to 0.25 wt.% Cr which renders an anodic potential noble.
AA 7072 Al alloy is relatively high is sacrificial anode property because of the Zn content in an appropriate amount. However, AA 7072 is very poor in erosion-corrosion resistance since said alloy contains only up to 0.10 wt.% Mg and only up to 0.10 wt.% Mn as impurities.
AA 7472 Al alloy contains Zn and Mg in appropriate amounts. However, AA 7472 Al alloy is insufficient in sacrificial anode property since said alloy contains up to 0.25 wt.% Si which makes an anodic potential noble. Furthermore, AA 7472 Al alloy is insufficient also in erosion-corrosion resistance because of an Mn content of only up to 0.05 wt.% as an impurity.
In the U.S. patent application Ser. No. 860,571 of Dec. 14, 1977, now U.S. Pat. No. 4,141,725 there is described an aluminum alloy for sacrificial anode which consists essential of, in weight percentage:
Zinc: from 0.5 to 10.0%, PA1 Indium: from 0.005 to 0.05%, PA1 Calcium: from 0.005 to 0.5%, PA1 Magnesium: from 0.1 to 4.0%, PA1 and PA1 the balance aluminum and incidental impurities; PA1 Zinc: from 0.5 to 8.0%, PA1 Manganese: from 0.5 to 1.5%, PA1 and, PA1 the balance aluminum and incidental impurities; PA1 Zinc: from 0.3 to 3.0%, PA1 Magnesium: from 0.2 to 4.0%, PA1 Manganese: from 0.3 to 2.0%, PA1 and, PA1 the balance aluminum and incidental impurities; PA1 Indium: from 0.005 to 0.2%, PA1 Tin: from 0.01 to 0.3%, PA1 and, PA1 Bismuth: from 0.01 to 0.3%; PA1 provided that the total content of indium, tin and bismuth being up to 0.3%. PA1 Zinc: from 0.3 to 3.0%, PA1 Magnesium: from 0.2 to 4.0%, PA1 Manganese: from 0.3 to 2.0%, PA1 and, PA1 the balance aluminum and incidental impurities; PA1 Indium: from 0.005 to 0.2%, PA1 Tin: from 0.01 to 0.3%, PA1 and, PA1 Bismuth: from 0.01 to 0.3%, PA1 provided that the total content of indium, tin and bismuth being up to 0.3%; PA1 (1) Zinc:
said alloy including an aluminum alloy for sacrificial anode also containing at least one rare earth metal of from 0.001 to 0.05 wt.%.
The above-mentioned aluminum alloy shows an excellent sacrificial anode property even in low-temperature sea water. However, not only said alloy contains from 0.005 to 0.5 wt.% calcium (Ca) detrimental to erosion-corrosion resistance, but also no Mn is contained, leading to a problematic erosion-corrosion resistance.
In the U.S. patent application Ser. No. 910,212 of May 30, 1978, now U.S. Pat. No. 4,150,980 an aluminum alloy excellent in high-temperature sagging resistance and sacrificial anode property, which consists essentially of, in weight percentage:
said alloy including an aluminum alloy excellent in high-temperature sagging resistance and sacrificial anode property, which also contains from 0.01 to 0.5 wt.% zirconium (Zr).
The aforementioned aluminum alloy shows an excellent sacrificial anode property and an excellent high-temperature sagging resistance. However, because of the absence of Mg, said alloy has a poor erosion-corrosion resistance.
Under such circumstances, there is a keen demand for the development of an aluminum alloy excellent not only in sacrificial anode property but also in erosion-corrosion resistance, which is adapted to be cladded onto the inner surface of a pipe for cathodic protection thereof.